Canberra Raiders coach Ricky Stuart has launched a scathing spray at the referees after Sunday's 26-20 loss to Penrith, branding their display ''not up to first grade standard'' and ''laughable''.

Trailing 26-16 midway through the second half, the Raiders were fuming when a try to Shaun Fensom was disallowed, despite Penrith's James Segeyaro later being placed on report for an alleged forearm to David Shillington's head.

The Panthers were originally given a penalty before the play unfolded after Canberra prop Shillington lashed out at Segeyaro when playing the ball.

Fensom then scored off the next play, but the referees Gavin Badger and Grant Atkins waved it off after awarding a Panthers penalty, before reversing the call and penalising Segeyaro.

Canberra fans lined the tunnel to boo the referees after the controversial and sometimes spiteful affair at Canberra Stadium.

Already on edge after huge back-to-back losses to Manly and New Zealand, Stuart vented his frustration at the officials.

"I've been involved in some clangers in my career and I'll have you critics lining up to say I'm a whinger, but that takes the cake today,'' Stuart said.

"I've seen some good ones, but gosh that was poor.

''The referees today were not at the standard of first-grade referees.''

How the NRL views Stuart's comments will be watched with interest.

He was fined a total of A$25,000 (NZ$27,000) for two incidents of criticising referees when at Parramatta last year.

The Fensom no-try was a huge turning point in a drama-filled second half, including Canberra five-eighth Terry Campese accusing a Penrith player of a head-butt in the lead-up to a Panthers try.

Campese lost his cool with Badger, declaring "(he) f***ing head butts me, you've done nothing the whole game about them slowing the play the ball down.''

It was one of many heated exchanges between frustrated players and the referees, with a litany of swear words broadcast live on national television through the referees' microphones.

"The reason there was two blues is the players are frustrated, the referees caused those blues,'' Stuart said.

"Campo gets frustrated, he's a competitor and he's a frustrated player at the moment.

"When you get poor refereeing decisions that are just incorrect, purely incorrect, it frustrates you.

"The most important spectator at that game today was Archer [referees' boss Tony Archer] - he saw it first-hand.''

With Penrith on top 26-20 with 90 seconds left, controversy reigned again when the Panthers were handed a penalty for winger David Simmons being tackled in mid-air when he dropped a bomb.

The referees originally awarded Canberra a scrum 10 metres out, but reversed the decision after watching a replay.

"That last one was laughable,'' Stuart said.

Canberra showed vastly improved spirit and attitude compared to its embarrassing last-start defeats to the Sea Eagles and Warriors.